System pressure rises when heating on and it seems not the expansion tank?

[paltibine]

A few months ago we had our Baxi combi 133 HE Plus moved and since then I've had to bleed the rads in the loft conversion a couple of times and then top up the pressure in the system. All seemed to be fine and making sense.
The other night I noticed the system pressure very low (in the red) and opened the filling loop to top to 1.5 bar in the middle of the green section. About half an hour later the pressure was right at the top of the gauge and the pressure relief valve had opened sending hot water out of the overflow. I assumed I hadn't shut the filling loop properly and disconnected it and let out some water to take pressure back to about 2 bar (as it was hot). Next time the heating came on the pressure shot up again so I checked the expansion tank pressure and it seemed OK so I let it cool down and let out some more water. Pressure rises again and water comes out of overflow.
Then I checked the expansion tank carefully and bled out all the air (no water came out) and let the system pressure down to nothing. Then I pumped up the air side to 1bar (it says on the side of the tank) and opened the filling loop to take system pressure up to 1.5 bar. When pumping the air side there was no bubbling or strange noises and I could see the pressure rising on my pump gauge and on the system gauge.

But still when the heating comes on the pressure rises up to the red section (3 bar or so) and the release valve starts to drip!
I can't see why the expansion tank isn't working as it used to when the seal seems to be intact? What else could it be? Is there anything else I can check?

 

[Simon F]

could be a blockage in the pipe to the expansion vessel, you should really call an GSR engineer out to check it over

 

[Puddle]

Seems like maybe the small tube connecting expansion vessel to boiler inners could be blocked up

ps....when you pump up expansion vessel,water should be drained out of boiler and drain point left open

 

[casaco]

Like puddle says, when you recharge the pressure vessel the drain point must be open, the fact that your boiler pressure gauge indicated an increase in pressure while you were putting air in would suggest that the pressure vessel has water in it. Normally your boiler gauge would be at zero even with the vessel pumped up to 1 bar because the air in the vessel is remote from the pressure of the water in the system supplied by the filling loop.

 

[paltibine]

Thanks for the replies. I will get it looked at by an expert but I'm just trying to understand what could be wrong.

Normally your boiler gauge would be at zero even with the vessel pumped up to 1 bar because the air in the vessel is remote from the pressure of the water in the system supplied by the filling loop.

I'm not sure I understand this. I thought the pressure vessel consisted of air on one side of a diaphragm and the other side full of water from the heating system. The air in the pressure vessel is not really remote from the water in the system, just separated from it by the diaphragm. Pressure changes on one side of the diaphragm will be seen on the other side!

I pressurised the pressure vessel to 1 bar with the system closed so the air I added pressurised the water in the system. If I had done it with the water system open it would have taken a lot more air to get the same pressure! That would provide more of a buffer when the water heats up and starts to expand and so may be the problem!

Is there a way to isolate the boiler and open a drain point in the boiler without draining down the entire system? If I want to get 1bar on the air side with no pressure on the water side, could I do it by pumping up the air side then allowing water out of the system to take the pressure down and repeating the process until I have 1bar on the air side with zero pressure on the water side?

 

[casaco]

Most baxis have a drain point on right hand side of boiler, (little white knob), just close flow and return valves to boiler, open drain tap and catch in a bucket, make sure boiler is electrically isolated and cover pcb. when the water has finished draining pump the vessel up to 1bar, you will probably get more water coming out of the drain tap. When boiler completely empty, close drain tap open return valve and fill up using filling loop, open flow and make sure your bleed the pump and check if cap on auto vent is loose. DONT forget to open flow valve. See if that cures the problem.
I fix baxis every week, loads of them have got a secondary expansion vessel fitted, becuase the original has failed.

 

[casaco]

Just had a quick look at the manual for this boiler, you don't come across many of these boilers where I live for some strange reason, but it does say thet there are four drain points, it doesn't matter which one you use, but you must go to great lengths to make sure that not even 1 drop of water gets onto the pcb, use the lowest drain point you can on the boiler, it looks like the flow and return valves are fitted with drain points, turn the valves off first though. And the power.

 

[paltibine]

Well it's a bit soon to be sure but it's looking good!

In the end I just let the water pressure out, pumped the air side up to 1 bar, let more water out, pumped up to 1 bar again and repeated this a couple of times. When the pressure vessel was at 1bar with no pressure showing on the system gauge I connected up the filling loop and topped the water up to 1.5 bar and tried it. After about 40 mins with the heating on the pressure gauge was still in the green showing about 2bar.

The only slight problem is that there now seems to be a slow drip from the pressure release valve even at 2 bar which I've read is common once the release valve has been tripped once it will never seat quite properly again and should be replaced. Still it's cheaper than a new pressure vessel!

So thanks very much for all your help!